Introduction: The Fluorescent Lamp
Over 500 million fluorescent lamps are sold in the United States every year. Sales of “fluorescent lumiline lamps” commenced in 1938, when four different sizes of tubes were introduced to the market. During the following year, General Electric and Westinghouse publicized the new lights through exhibitions at the New York World's Fair and at the Golden Gate Exposition in San Francisco. Fluorescent lighting systems spread rapidly during World War II, as wartime manufacturing intensified lighting demand. By 1951, more light was produced in the United States by fluorescent lamps than by incandescent lamps.
How a Fluorescent Lamp Works
FIG. 1 depicts a generalized version of a conventional fluorescent lamp, which comprises a sealed glass tube filled with a gas that is maintained at very low pressure. When the gas is excited by applying an electrical current across the ends of the tube, particles generated by the excited gas strike the coating on the inside of the tube, and the coating emits visible light. (See GE Lighting, How It Works and Westinghouse Light Bulbs websites.)
A generalized pictorial view of a fluorescent lamp is depicted in FIG. 2. The fluorescent lamp is usually filled with a gas containing low pressure mercury vapor and argon, xenon, neon, or krypton. The pressure inside the lamp is around 0.3% of atmospheric pressure. The inner surface of the bulb is coated with a fluorescent (and often slightly phosphorescent) coating made of varying blends of metallic and rare-earth phosphor salts. The tube has two electrical terminals, a cathode and an anode. The cathode is typically made of coiled tungsten. This coil is coated with a mixture of barium, strontium and calcium oxides (chosen to have a relatively low thermionic emission temperature). When the light is turned on, the electric power heats up the cathode, and it begins to emit electrons into the lamp enclosure. The mercury atoms in the fluorescent tube must be ionized before the arc can “strike” within the tube. The electrons emitted from the cathode collide with and ionize noble gas atoms in the bulb surrounding the filament, and form a plasma by a process of impact ionization. The ultraviolet light is absorbed by the bulb's fluorescent coating, which re-radiates the energy at longer wavelengths to emit visible light. (See Wikipedia.)
A conventional incandescent light is shown in FIG. 3. An electrical current flows through a metal filament in an evacuated glass bulb. The electricity heats the wire filament, which produces a glow of visible light. A conventional incandescent light bulb is “electrically stable,” meaning that when the bulb is turned on, current flows through the filament at a relatively steady rate, and light is produced until the bulb is turned off. A fluorescent tube, by itself, is “electrically unstable.” When power is applied to an uncontrolled fluorescent light, more and more power flows into the lamp, and, eventually, the lamp burns up and is destroyed. This unfortunate result is due to an electric characteristic of the fluorescent lamp, which is based on the electrical property called “resistance.” In general, resistance is a characteristic of a substance to carry or convey a flow of electricity. Metals, like copper, gold and silver, are the best conductors of electricity, and have relatively low resistance. Insulators, like glass or plastics, do not allow electricity to pass, and, in general, have a relatively high resistance.
In a conventional incandescent light bulb, the resistance of a heated filament is relatively constant. In other words, once power is applied to a conventional light bulb, the filament heats up, and the amount of electricity that flows through the bulb remains about the same until the power is switched off.
In a conventional fluorescent lamp, after the power is initially supplied to the electrodes of the fluorescent lamp, the gas inside the tube is excited, and its electrical resistance begins to fall. More electricity flows into the lamp when the resistance drops, and the cycle continues unabated until so much current flows into the lamp, that the lamp is destroyed by excessive heat.
Controlling the Fluorescent Lamp: The Ballast
The operation of conventional fluorescent lamps may be controlled by using an external device, called a “ballast,” which limits and regulates the current flow through the tube. The ballast may be a simple electrical component called a “resistor,” which limits the flow of energy into the lamp. A more prevalent form of ballast employs another electrical component called an “inductor,” which generally comprises a coil of wire wrapped around a metal core. Many different circuits have been used to start and run conventional fluorescent lamps. The design of a conventional ballast is based on input power voltage, tube length and size, initial cost, long term cost and other factors. (See Wikipedia).
Supplying Power to a Fluorescent Lamp
Conventional fluorescent lamps may be powered by a direct current (DC), which flows in a steady stream, and which does not vary with time. In DC powered fluorescent lamps, the ballast must be resistive, and consumes about as much power as the lamp. Current day fluorescent lamps are almost never powered by direct current. Instead, the vast majority of present day fluorescent lamps run on alternating current (AC), which rises and falls in a regular cycle. FIGS. 4 and 5 furnish two graphs that compare direct current and alternating current.
More recent “electronic” ballasts utilize transistors or other semiconductor components to convert household voltage (120 VAC) into high-frequency alternating current.
Beginning in the 1990's, a new type of ballast was introduced to the market. “High frequency” ballasts use high frequency voltage to excite the mercury within the lamp. These newer electronic ballasts convert the 60 Hertz household alternating current to a high frequency signal that can exceed 100 kHz. (See Wikipedia).
Present day conventional ballasts and fluorescent lamps are hampered by serious limitations. First, they consume substantial amounts of power. Second, they are not dimmable over a complete range of brightness. Third, every ballast must be especially configured for the particular fluorescent lamp with which it is to be used.
The development of an energy control device system that overcomes these limitations and that provides a substantial reduction in energy consumption would constitute a major technological advance, and would satisfy long felt needs and aspirations in the lighting industry, and would also satisfy pending and imminent regulatory demands.